First, the reassuring part: if a buy cannot be completed, your Naira is automatically refunded to your balance, and you will get a "Buy failed" notification letting you know.
Why a buy sometimes fails
Slippage: the token's price moved too much between quote and execution. Please try again.
Not enough liquidity right now: try a smaller amount, or try again a little later.
The quote expired before it could be completed. Just start a fresh one.
A network fee issue stopped the trade. Try again.
If you were debited but you can see neither the crypto nor a refund after some time, reply with your Celia Cash username, the coin, network, amount, and time, and the transaction hash if you have one, so we can reconcile it.